1. Field of the Invention
This invention relates to methods of manufacturing polymeric medical devices, in particular, stents, and especially stents used in the treatment of blood vessels.
2. Description of the State of the Art
Until the mid-1980s, the accepted treatment for atherosclerosis, i.e., narrowing of the coronary artery(ies) was by-pass surgery. While effective and evolved to a relatively high degree of safety for such an invasive procedure, by-pass surgery still involves potentially serious complications, and in the best of cases an extended recovery period.
With the advent of percutaneous transluminal coronary angioplasty (PTCA) in 1977, the scene changed dramatically. Using catheter techniques originally developed for heart exploration, inflatable balloons were employed to re-open occluded regions in arteries. The procedure was relatively non-invasive, took a relatively short time compared to by-pass surgery, and the recovery time was minimal. However, PTCA brought with it other problems such as vasospasm and elastic recoil of the stretched arterial wall which could undo much of what was accomplished and, in addition, it created a new disease, restenosis, the re-clogging of the treated artery due to neointimal hyperplasia.
The next improvement, advanced in the mid-1980s, was the use of a stent to maintain the luminal diameter after PTCA. This for all intents and purposes put an end to vasospasm and elastic recoil, but did not entirely resolve the issue of restenosis. That is, prior to the introduction of stents restenosis occurred in about 30-50% of patients undergoing PTCA. Stenting reduced this to about 15-20%, much improved but still more than desirable.
In 2003, drug-eluting stents or DESs were introduced. The drugs initially employed with the DES were cytostatic or cytotoxic compounds, that is, compounds that curtailed the proliferation of cells that contributed to restenosis. The occurrence of restenosis was thereby reduced to about 5-7%, a relatively acceptable figure. Thus, stents made from biostable or non-erodible materials, such as metals, have become the standard of care for percutaneous coronary intervention (PCI) as well as in peripheral applications, such as the superficial femoral artery (SFA), since such stents have been shown to be capable of preventing early and later recoil and restenosis.
However, a problem that arose with the advent of DESs was so-called “late stent thrombosis,” the forming of blood clots long after the stent was in place. It was hypothesized that the formation of blood clots was most likely due to delayed healing, a side-effect of the use of cytostatic drugs. One potential solution is to make a stent from materials that erode or disintegrate through exposure to conditions within the body. Thus, erodible portions of the stent can disappear from the implant region after the treatment is completed, leaving a healed vessel. Stents fabricated from biodegradable, bioabsorbable, and/or bioerodible materials such as bioabsorbable polymers can be designed to completely erode only after the clinical need for them has ended. Like a durable stent, a biodegradable stent must meet time dependent mechanical requirements. For example, it must provide patency for a minimum time period.
Thus, there is a continuing need for biodegradable stents that meet both mechanical requirements, and methods of forming such stents.